Production of distillate



P 1944- A. D. GARRISON PRODUCTION OF DISTILLATE Filed Nov. 27, 1941ALLEN D. GARRISON W Aw HIS ATTORNEYS OZUDOOwE Patented. Sept. 26, 1944PRODUCTION OF ms mars Allen D. Garrison, Houston, Tex, assignor toTexaco Development Corporation, New York,

N. vIL, a corporation of Delaware Application November 27, 1941, SerialNo..420,675

11 Claims. This invention relates to a process for the production ofliqueflable hydrocarbons from fluids flowed from distillatereservoirs,,involving removing a distillate fluid from one or moreproducing wells, recovering liqueflable hydrocarbons from the fluid andreturning residual gases to the same or a different producing formation.

The producing formations of distillate fields are characterized inthat-the hydrocarbons are considered to be present iii the sand as asingle homogeneous phase. The known distillate formations are usually atdepths of 8,000 feet or more and the bottom hole pressures are normallyfrom 3,000 to 5,000 pounds per square inch or above. As deep drillingcontinues it may be expected that distillate formations will bediscovered at increasingly higher pressures.

Distillate fluids are'distinguished from other fluids in producingformations in that they possess the property of undergoing condensationupon an isothermal reduction in pressure; 1. e., they are subject toretrograde condensation. To avoid the operation of this phenomenon inthe formation, and to comply with State laws in regard to conservation,it is necessary to maintain formation pressures by returning theresidual gases to the formation. As a result of this requirement, theprocesses which have been proposed have embodied attempts in one way oranother to recover the liqueflable hydrocarbons without reducing thepressure any more than necessary. However, because the maximumcondensation of hydrocarbons has apparently taken varying from about1,000 to 2,000 pounds per square inch, depending upon thecharacteristics of the particular fluid, the processes in actualoperation have involved reducing the pressure to some pressure below2,000 pounds per square inch.

Residual gases, therefore, have been obtained at a low pressure relativeto the formation pressure and compression costs have been high.

Perhaps the simplest process that has been employed in the production ofdistillate has consisted in flowing the distillate fluid from theproducing well through a choke to reduce the pressure to thedesired-point, cooling the fluid in one or more place at pressuresrepresent an important part of the costs of operating a process of thistype.

It is an object of the present invention to provide a simple andeflicient process for the production of a distillate fleld, whichprocess is characterized in that compression costs are reduced, and theamount of processing of the fluid is lessened.

Other objects of the invention in part will be obvious and in part willappear hereinafter.

In accordance with the present invention the production of distillate isaccomplished by a process in which a high density liquid is mixed withthe distillate fluid in the producing well, preferably at about thedepth of the producing formation, and the resulting mixed fluid ispermitted to flow from the well due to the pressure of the formation.The mixed fluid is then passed from the well and is treated to separatethree fractions:

the high density liquid, the desired liqueflable hydrocarbons, whichpreferably consist of most of the butanes and substantially all of thepentanes and heavier, and the residual lighter gases. The separationstep is preferably carried out at about the pressure at which the mixedfluid is removed from the producing well, a pressure materially belowthe bottom hole pressure and of course below the pressure to which theresidual gases would have to be raised to make possible their re steps,separating liqueflable hydrocarbons which are condensed under theseconditions, and compressing and returning the residual gases to the sameor a different producing formation. It will be seen that where theformation pressures are in the neighborhood of 4,000 pounds per squareinch and the condensation of hydrocarbons is accomplished at a pressureof the order of 1,200 to 1,500 pounds per square inch, the costs ofcompression turn to a high pressure formation.

-In the case where the residual gases are to be returned to the sameproducing formation, after the separation of the mixed fluid, thepressure on the residual gases at the surface is raised arelativelysmall amount (e. 8.. 1,000 to 1,200 pounds per square inch)'above well-head pressure. At

the same time the high density liquid recovered in the separation israised to the same pressure.

The high densityiiquid and the residual gases are then mixed and passedinto the injection well. Atthe bottom of the well, preferably oppositethe formation, the residual gases and the high density liquid arepermitted to separate. and the gases pass into the formation to maintainthe pressure. The high density liquid is removed from the injection welland employed for admixture with disq tillate fluid in the productionwell in the manner described above.

While the mechanism and manner of operation of the process and theadvantages obtained will be more or less obvious as the descriptionproceeds, it may be of advantage to point out here the broad-principleupon which the process operates.

" When flowing a distillate well in the normal way there is, of course,a reduction in the pressure of finely divided form. However, I prefersurface before introducin This reduction in pressure is due to twofactors, the pressure of the column of fluid and friction of the tubing.By mixing the distillate fluid in the producing well with a high densityliquid the difference in pressure between the formation and thewell-head may be adjusted to .any desired value. This is done bycontrolling the height and/ or density of the mixed fluid and therebythe pansion of distillate fluids to recover liquefiable,

hydrocarbons by a method which stores the energy of expansion inrecoverable form. The recoverable energy is used to repressure thereservoir. There are also thermal advantages which result. The heavyliquid injected into the producing well and mixed with the well fluidserves also as a cooling agent. There is added to this the coolingeffect of expansion of the gas while doing work in lifting the heavyliquid to the surface, and delivering it at a substantially increasedpressure. With certain types of distillates, this cooling together withproper pressure reduction will provide satisfactory hydrocarbonrecovery. with other cases where additional cooling or absorption becomedesirable, their incorporation in .this improved method becomes moreeconomical than would usually be the casesince substantial cooling isalready an inherent feature of the process.

In addition to saving in power, a major feature of this inventionconsists of the elimination of high pressure surface equipment. Whenusing the residual gases for repressuring purposes the introduction ofthe high density liquid into the gases produces a relatively dense mixedfluid which at a relatively low surface pressure will produce a highbottom hole pressure in the injection well. As a result; the necessityfor compressing the residual gases to a high pressure at the surface iseliminated.

It will be understood in view of the variable temperature employed atthe well head, which may be 32 F. "or lower. v

The salts which may be employed are any salts that produce a solution ofthe. desired density, and the selection of a salt solution in aparticular case may be made by reference to published informationregarding the properties of the solutions. Salts which are of particularinterest for use in making up the high density liquids are halides,nitrates and sulfates of various metals which yield water-soluble saltswith the acid radical. Specific examples of such salts are calciumchloride, zinc-chloride, zinc bromide, zinc iodide,

barium iodide, and cadmium chloride. Also of conditions encountered indistillate production that various types of high density liquids may beused in the present process. In general it is preferred to employ aliquid in which hydrocarbons are substantially insQuble, although it isconsidered that in certain cases relatively heavy hydrocarbon oils maybe found of advantage. It is also considered that in certain casesit.may be found desirable to employ as the high density liquid water oroil weighted with heavy solids in to employ a water solutionof a heavymaterial. As suitable heavy materials inorganic salts may be mentioned.In selecting a particular salt one factor which has not yet beenreferred to should be taken into account. As noted above, on flowing themixed fluid from the formation to the head of the production well thetemperature of the liquid is reduced. The final temperature em- Ployedat the well head may be reduced still further by cooling the highdensity liquid at the it into the production well. Accordingly, inselecting a salt solution, a solution should be selected which has afreezing point or point of separation of solute below the interest arepotassium-chrom-alum, K2Cr2(SO4)4 and soda alum, NazSO4.Al2(SO4)a.24H2O.It will be understood that in order to achieve the desired density andfreezing point a solution containing two or more salts may be used. Forexample, salts such as sodium bromide, sodium idide, sodium nitrate orpotassium carbonate may be used in conjunction with soda alum. Sodiumsilicate solutions are also of interest for use in the present process.

In general, it is preferred to employ a liquid having a density of above1.2 grams per cc. ordinarily the heaviest liquid that will operate beingpreferred. Because of ready availability, cheapness, andnon-corrosiveness, solutions of calcium chloride are of particularinterest. Aqueous solutions of this salt can be prepared to havesufficient density and 'yet be stable at relatively low temperatures. Toillustrate the properties of solutions of calcium chloride the followingtable is given. In this table the minimum temperature is the temperatureat which separation of CaCl: from solution takes place.

Specific M iuimum pewent C801 gravity temperature In order that theinvention may be understood more fully the process will be described inconnection with the accompanying drawing which represents indiagrammatic form a production well, an injection well, and above-groundapparatus elements employed in carrying out the present invention inaccordance with one man.- ner of proceeding.

The drawing illustrates a relatively simple apducing well which taps adistillate formation. ,For the purposes of this description the wellwill be considered as being 8,000 feet in depthwith a bottom holepressure of about 4,200 pounds per square inch. The well A is providedwith a casing l0 which contains perforations I2 opposite the producingsand H. Disposed in the well is a flow tubing it which is provided withan open a single injection well.

' injection well.

end l3. Disposed about the flow tubing I6 is a concentric tubing 22 oflarger diameter. The tubing 22 is joined to the flow tubing with a fluidtight joinder at 24. Leading from the top of tubing 22 is a vent pipe 23provided with a valve 25. Just'above the joinder at 24 the flow tubingis provided with perforations 26. A valved line 23 leads from the top offlow tubing l6 to a separator 30.

The separator is of such volume and is so constructed as to permit theseparation of three phases therein: residual gases, distillate, and thehigh density liquid. The residual gases may be passed from the separatorthrough a valved-line 32 directly to a compressor 34 and thence throughline 36 into an injection well B. However, where the desired percentage01' liqueflable hydrocarbons is not recovered in the separator, theremay be placed between the separator and the compressor a recoverysystem, indicated generally at II, which system may be an absorption oradsorption system or a system wherein the gases are subjected toadditional cooling to remove additional distillate therefrom. Thedistillate may be removed from this system through valved line 33leading to line 35 described below, and the residual gases are thenpassed to the compressor 34.

The high density liquid is removed from the bottom portion of theseparator through a valved line 36 leading to a pump 40 and thencethrough line 42 and at point 43 into line 36 leading to the Distillateis removed from the separator through valved line 44, passes into line36 and thence into distillate storage vessel 46. A line 46 provided withvalve 49 leads from a pump 50 to the interior of tubing 22 in producingwell A.

The injection well B is constructed so as to carry out the operation ofthe present process in the following manner. Similarly to well A, it

is provided with a casing 52 which has perforations 64 opposite theproducing formation. Within this casing there is an inner casing ortubing 36 having a diameter only slightly less than the diameter of theouter casing, and having a closed end 58. Disposed within this innercasing is an injection tubing 62 connected by means of valve 64 to line36. A second tubing 66 is disposed within the tubing 62 and leads tovalved line 63 connected with pump 50.

At some distance from the bottom of the well, the inner casing 56 isprovided with perforations 12, by means of which gases may pass from theinner easing into the space between the. two casin s and into theformation. It will be noted that the outer and inner casings define anenclosed annulanspace which is open only at the perforations 12 and 54.As will appear from the following description, the perforations I2 maybe located in some cases more than half way from the bottom of the wellto the surface.

In carrying out the process in an apparatus of the'type illustrated inthe drawing, a high density liquid may be selected such that a column ofthe liquid equalin length tothe depth of the well, or, where the liquidis not introduced into the bottom of the well, equal to the distancefrom the surface to the point of introduction, will exert a pressureonly slightly above the pressure existing in the flow tubing at thepoint of introduction. Only a suiflcient difierence in pressure isrequired to cause the high density liquid to flow into the flow tubing.

The process may also be carried out using a liquid of higher density andregulating the height of the column to yield the desired pressure atthe-point of introduction. By using a higher density liquid, thepressure of the column of liquid in the injection wellmay be severalhundred pounds above the pressure existing in the bottom of theinjection well. This latter manner of operating will be particularlydescribed. In the present case the high density liquid used is a 34.5per cent solution of calcium chloride which has a density of about 1.34grams per cc.

A column of this liquid extending from the top to the bottom of well Bwill exert a pressure slightly less than 4700 pounds per square inch.

The process is begun with the tubing 22 filled to a height of about 7200feet with the calcium chloride solution which is fed in at a constantrate through line 48, trapped gas being released through line 23. Thefluid flowing from formation i4 into the end l8 offlow tubing l3 passesthe perforations 26 and there comes into intimate contact with thecalcium chloride solution. The calcium chloride solution is fed in at arate controlled by the size of the perforations 26 and the head of about7200 feet .in tubing 22 such that the solution is mixed with thedistillate fluid in the proportion of about equal'parts liquid and fluidby volume. Since the fluid may have an average density of, for example,.20 gram per cc., and the liquid 9. density of 1.34 grams per cc. amixed fluid may be produced having an average density of about .77 gramper cc. Due to the density of this mixed fluid and the frictional lossthe well-head pressure may be of the order of 1200 pounds per squareinch. Also, due to the reduction in pressure of the fluid while doingwork, and the cooling action of the liquid, which may be increased bycooling the liquid before introducing it into the well, the temperatureis at a low point at which effective condensation of liqueiiablehydrocarbons occurs; e. g., at a temperature of about 50 to F.

In this operation, while the distillate fluid expands from about 4200pounds per square inch gage to about 1200 pounds per square inch, thehigh density liquid is compressed from 0 pounds per square inch to 1200pounds per-square inch. Under these conditions the mixed fluid is flowedthrough line 23 to separator 30. The residual gases ar removed from theupper portion of the separator through line 32, and, if no further.

treatment is necessary, are. passed directly to compressor 34 where theyare compressed to a pressure of, for example, 2400 pounds per squareinch The desired liqueflable hydrocarbons or distillate are removedthrough line 44 and are collected in distillate storage vessel 46. Thehigh density liquid is removed from the bottom of the separator throughline 33 to pump 40, wherein the pressure on theiiquid is raised toabout2400 pounds per square inch, andpassed through line 42, and, at point43, into admixture with the residual gases flowing in line 36.- Theresulting mixed fluid is then passed through tubing, 62 into theenclosed bottom portion of inner casing 56 which forms a zone 53. Sincethis mixed fluid may have an average density of the order of .76 gramper cc., the pressure in the inner casing 56 in the neighborhood of theformation may be, for example, 4700 pounds per square inch. Under thequiescent conditions existing in zone 59, the

' gas will separate from the high density liquid and will pass up andthrough the perforations in inner through line 48, valve and back intotubing 22.

' In certain cases it be found desirable to subject the mixed fluidflowing in line 28 to a limited amount of cooling. This may be done soas to replace the recovery system 3| or in addition to this system.Although the temperature of the mixed fluid at the well head will bebelow the temperature of ordinary hydrate formation, because of thepresence of the calcium chloride or other salt solution no dillicultiesdue to the formation of hydrates will be experienced.

As previously indicated, in some cases it may be necessary to employ arecovery system in addition to the separator. The general operation ofthe process in such case will not differ materially from that described.i

It will be noticed that it is necessary to compress the residual gasesto only a few hundred pounds above the pressure at the well-head, thussimplifying compressor equipment and reducing power costs for thisoperation. With regard to the extent of this rise in p'ressure,'thiswill be dependent upon the density and amount of high density liquidused, the depth of the well, and the formation pressure. In any case.the pressure of the residual gases after compression will be far belowthe normal well-head pressure of the production well, and of course evenfarther below the pressure normally required to return. residual gasesthrough an injection well to the formation.

In selecting the high density liquid it is generally preferred to selecta liquid such that the hydrostatic head 'of the liquid in the injectionwell is about equal to the repressuring pressure 1. e., the repressuringpressure less the pressure lost by friction. Such a liquid will flowunder easily controlled conditions to the producing well. Also, the flowof the liquid into the distillate fluid in the producing well may becontrolled easily by. regulation of the height of the column of liquidand the size of the perforations 26. In this connection, the size ofthese openings may be made adjustable; for example, by providing meansfor making tubing 22 rotatable relative to tubing l6 and means operatedby the relative rotation to reduce or enlarge the openings.

When using a relatively high density liquid, the amount of the liquidintroduced into the distillate fluid is controlled so as to obtainthedesired well-head pressure. For example, using a 50 per cent solution ofzinc chloride, which has a density of about 1.57 grams per cc., in theabove process, the height of the column of liquid in the producing wellwould becontrolled to maintain it at about 6700 feet. When using thissolution in an operation as described in the drawing, the solution wouldbe mixed with the distillate fluid in the proportions of about 40 percent of thesolution and about 60 per cent of the distillate fluid byvolume.

As indicated above, the drawing herein is merely diagrammatic incharacter to show the flow of fluids and no attempt has been made toshow the elements in the relative sizes that would be used in practice.

From the foregoing it will be seen that the preferred operation of theprocess involves controlling the density or height or both of the high'density liquid so as to obtain a pressure at the bottomof thecolumn ofliquid in the producing well about equal to the pressure of thedistillate fluid. 'The rate of flow of the liquid is best adju'sted bycontrolling the pump. 50 to yield a mixed fluid of the desired density.The desired density of the mixed fluid is, of course, dependent upon thebottom hole pressure, the optimum pressure at the surface, and the depthof the well. In the normal case, conditions will be adjusted so that thepressure at the head of the producing well will be at about the bottomof the retrograde condensation range for the fluid in question. Thus,conditions may be adjusted so that th well-,-head pressure will bewithin the range of about 800 to 1600 pounds per square inch.

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore only such limitations should be imposedas are indicated in the appended claims.

I claim:

1. In the production of a distillate field wherein a distillate fluid isflowed from a producing well tapping the producing formation,liqueflable hydrocarbons are recovered from the distillate fluid. andresidual gases are returned through an injection well to a formation tomaintain pressures. the process which comprises mixing the distillatefluid in the producing well with a high density liquid, permitting theresulting mixture to flow to the surface, whereby the temperature andpressure of the distillate fluid are reduced and liqueflablehydrocarbons are condensed, separating from said mixture liqueflablehydrocarbons, residual gases, and said high density liquid, passing amixture of said residual gases and said high density liquid into theinjection well at a pressure above the pressure atthe head of theproducing well, separating said last-mentioned mixture in the injectionwell into said residual gases and said high density liquid, passing saidresidual gases into the formation to maintain pressures, and returningthe high density liquid to the producing well foradmixture with thedistillate fluid.

2. A process in accordance with claim 1 in which the high density liquidis a liquid in which thehydrocarbons of the distillate fluid aresubstantially insoluble.

- 3. A process in accordance with claim 1 in which the high densityliquid has a density of at least 1.2 grams per cc. and comprises anaqueous high density liquid into the producing well out I of contactwith the distillate fluid so as, to maintain a moving column of highdensity liquid therein, mixing the high density liquid with thedistillate fluid at substantially the depth of the formation,controlling the pressure at which the high density liquid is introducedinto the distillate fluid to a pressure adapted to cause the flow of thehigh density liquid but being. not substantially above the pressure ofthe formation, permitting the resulting mixture to flow to the surface,whereby the temperature and pressure of "the distillate fluid arereduced and liqueflable hydrocarbons are condensed, separating from saidmixture liqueflable hydrocarbons, residual gases and said high densityliquid, raising the pressure on said residual gases and on said highdensity liquid, mixing said residual gases and said high density liquidat the raised pressure, passing the resulting mixture into the injectionwell, separating said last-mentioned mixture in the injection well intosaid residual gases and said high density liquid, passing said residualgases into the formation to maintain pressures, and returning the highdensity liquid to the producing well for admixture with the distillatefluid.

6. In the production of a distillate field where-' in a distillate-fluidis flowed from a producing well tapping the producing formation,liqueflable hydrocarbons are recovered from the distillate fluid, andresidual gases are returned through an injection well to the formationtomaintain pressures, the process which comprises flowing a high densityliquidinto the producing well out of contact with the distillate fluidso as to maintain a moving column of high density-liquid therein, mixingthe high density liquid with the distillate fluid in the producing well,the density of the high density liquid being such that. the pressure ofthe column of liquid at the point of intro,- duction into the distillatefluid is slightly greater than the pressure on the fluid, and the proporf tions in which the distillate fluid and the high? returning the highdensity liquid to the producing 7 well for admixture with the distillatefluid.

8. In the production of a distillate field wherein a distillate fluid isflowed from a producing well tapping the producing formation,liqueflable hydrocarbons are recovered from the distillate fluid, andresidual gases are returned through an injection well to the formationto maintain pressures, th process which comprises flowing. a highdensity liquid in which the hydrocarbons of the distillate fluid aresubstantially insoluble into density liquid are mixed being such thatthe re- 5 suiting mixture will flow from the producing well at thesurface at a selected reduced pressure withinthe retrograde condensationrange of the distillate fluid, permitting the resulting mixture to flowto the surface, whereby the temperature and pressure of the distillatefluid are reduced and liqueflable hydrocarbons are condensed, separatingfrom said mixture liqueflable hydrocarbons, residual gases and said highdensity liquid, raising the pressure on said residual gases and on saidhigh density liquid, mixing said residual gases and said high densityliquid at the raised pres-- sure, passing the resulting mixture into theinjection well, separating said last mentioned mixture in the injectionwell into said residual gases and said high density liquid, passing saidresidual the producing well out of contact with the distillate fluidso'as to maintain a moving column of high density liquid therein, mixingthe high density liquid with the distillate fluid at substantially thedepth of the formation, controlling the height of the column of liquidto obtain a pressure of the column at the point of introduction into thedistillate fluid slightly greater than the pressure on the fluid,permitting the resulting mixture to flow to the surface, whereby thetemperature and pressure of the distillate fluid are reduced andliqueflable hydrocarbons are condensed, separating from saidmixtureuliquefiable hydrocarbons, resid-.

ual gases and said high density liquid, raising the pressure on saidresidual. gases and on said high density liquid, mixing said residualgases and said high density liquid at the raised pressure, passing'arating said last mentioned mixture in the injec- =the resultingmixture into the injection well, seption well into said residual gasesand said high gases into the formation to maintain pressures,

density liquid into the producing well out of contact with thedistillate fluid so as to maintain a moving column of high densityliquid therein, 'mixing the high density liquid with the distillatefluid at substantially the depth of the formation, controlling theheight of the column of liquid to obtain a pressure of the column at thepoint of introduction into the distillate fluid slightly greater thanthe pressure on the fluid,

permitting the resulting mixture to flow to the surface, whereby thetemperature and pressure of the distillate fluid are reduced andliqueflable density liquid, passing said residual gases into theformation to maintain pressures, and returning the high density liquidto the producing well for admixture with the distillate fluid, thedensity' of the high density liquid being such that the pressure of acolumn of the liquid substantially equal in height to the depth of theinjection well exerts a pressure substantially equal to the pressurerequired for passing said residual gases into the formation to maintainpressures, whereby the formation is repressured and the high densityliquid flows from the head of the injection well.

9. In the production of a distillate field wherein a distillate fluid isflowed from a producing well tapping the producing formation,liqueflable hydrocarbons are recovered from the distillate fluid, andresidual gases are returned through an injection well to the formationto maintain pressures, the process which comprises flowing a, highdensity liquid in which the hydrocarbons of the distillate fluid aresubstantially insoluble into the producing well out of contact with thedistillate fluid so as to maintain a moving column of high densityliquid therein, mixing the high density liquid with the distillate fluidin the producing well, permitting the resulting mixture to flow to thesurface, whereby. the temperature and pressure of the distillate fluidare reduced and liqueflable hydrocarbons are condensed, separating fromsaid mixture liqueflable hydrocarbons, residual gases and said highdensity liquid, Passing a mixture of said residual gases and said highdensity liquid at a raised pressure into the injection well, separatingsaid last-mentioned mixture in the injection well at about the depth ofbut out of contact with the formation into said residual gases and saidhigh density liquid, passing said residual gases to the formation tomaintain pressures, and returning the high density liquid to theproducing well for admixture with the distillate fluid.

10. In the production of a distillate field where- I density liquid inwhich the hydrocarbons of the distillate fluid are substantiallyinsoluble into the producing well out of contact with the distillatefluid so as to maintain a moving column of high density liquid therein,mixing the high density liquid with the distillate fluid in theproducing well, permittingthe resulting mixture to flow to the surface,whereby the temperature and pressure of the distillate fluid are reducedand liqueflable hydrocarbons are condensed, separating from said mixtureliqueflable hydrocarbons, re-

sidual gases and said high density liquid, passing a mixture 01' saidresidual gases and said high density liquid at a raised pressure intothe injection well, separating said last-mentioned mixture in theinjection well at about the depth of but out of contact with theformation into said residual ases and said high density liquid, passingsaid residual gases to the formation to maintain pressures, andreturning the high density liquid to the producing well .for admixturewith the distillate fluid, the density of the high density liquid beinsuch that the pressure of a column of the liquid substantially equal inheight to the depth of the injection well exerts a pressuresubstantially equal to the pressure required for passing said residualgases into the to tion to maintain pressures, whereby the formation isrepressured and the high density liquid flows from the head ofthe'injection ,well.

tact with the distillate fluid so as to maintain a moving column or highdensity liquid therein, mixing the high' density liquid with thedistillate fluid in the producing well, permitting the result- ,ingmixture to flow to thessurface, whereby the temperature and Pressure ofthe distillate fluid are reduced and liqueflabl hydrocarbons arecondensed, separating from said mixture liqueflable hydrocarbons,residual gases and said high density liquid, passing a mixture of saidresidual gases and said high density liquid at a raised pressure intothe injection well, separating said last mentioned mixture in theinjection well at about a depth of the formation into said residualgases and said high density liquid, passing said residual gases into theformation to maintain pressures,

and returning the high density liquid to the producing well foradmixture with the distillate fluid, the density of said high densityliquid being such that the pressure of a column of the liquidsubstantially equal in height to the depth of the in- Jection wellexerts a pressure substantially equal to the pressure required forpassing said residual gases into the formation to maintain pressures,

whereby the formation is repressur'ed and the high density liquid flowsfrom the head of the injection well at substantially atmosphericpressure.

ALLEN D. GARRISON.

